1. Field of the Invention
The present invention relates to a method and apparatus for applying various kinds of liquid compositions onto a travelling web in manufacturing photographic film, photographic printing paper, magnetic recording tape, adhesive tape, pressure sensitive recording paper, offset paper, etc.
2. Background of the Invention
In one method of continuous application a liquid composition is caused to flow out of a die through a slit of the die. The most used versions of such a method are a method of multiple coating disclosed in U.S. Pat. No. 2,761,791, a curtain coating method disclosed in U.S. Pat. No. 3,508,947, and an extrusion coating method disclosed in U.S. Pat. No. 3,526,528.
The method according to the present invention is of course applicable to those methods described above, and is further applicable to various combinations thereof, such as extrusion coating method with a slide application, curtain coating method using a die for extrusion coating, etc.
Heretofore, a very important technique has been to manufacture coated products of even quality at an improved coating speed in the photosensitive material manufacturing process, in the process magnetic recording media manufacturing process, etc., in which high coating quality is required.
A problem in such an attempt of improving the coating speed in a high quality coating process is a phenomenon called "air entrainment". This phenomenon is caused such that the air existing on the surface of a travelling web flows to a dynamic contact point of coating at a speed equal to the speed of the travelling web so that the air partially enters between the applied liquid and the web to thereby form residual bubbles. Such bubbles are fatal defects in photosensitive material or magnetic recording media. Accordingly, prevention of occurrence of this phenomenon is one of the important problems to be solved in making the coating speed high.
The air entrainment phenomenon has been investigated not only from the industrial point of view but also from the scientific point of view as developed from the study of dynamic wetting at a liquid-solid junction.
In an article by R. Burley and B. S. Kennedy, entitled "Dynamic Wetting and Air Entrainment at a Liquid- Solid-Gas Junction" (Wetting, Spreading and Adhesion, Chapter 15, edited by J. F. Padday, Academic Press, 1978), it is described that the impinging speed V.sub.af (cm/sec) at which air entrainment occurred was measured by impinging a 2.5 cm wide polyester tape into liquid perpendicularly to the liquid surface, and it was found that the impinging speed V.sub.af was represented by the equation EQU V.sub.af =67.68(.mu.g/.rho..sigma.)-0.672
where .mu., .sigma., and .rho. represent the liquid viscosity (poise), the liquid surface tension (dyne/cm), and the liquid density (g/cm.sup.3), respectively. It is apparent from the equation that the occurrence of air entrainment can be prevented by making the liquid viscosity low. In fact, this method is important also in the industrial point of view. A method for reducing the viscosity of an applied liquid which comes into contact with a web after the liquid is applied to the web is disclosed in U.S. Pat. No. 4,001,024.
However, the method has a disadvantage that dilution of the liquid with a solvent is necessary for any remarkable reduction in the liquid viscosity and therefore excessive drying capacity is correspondingly required.
In another industrial method in which a decompression chamber is provided in a slide coating apparatus, as disclosed in U.S. Pat. No. 3,735,729, the effect of preventing air entrainment is hardly improved even if the degree of pressure reduction is increased beyond a certain value, but destruction of beads occurs owing to the intensive pressure reduction or stripes are produced owing to instability. Accordingly, the method is not satisfactory for keeping quality high with the increase of coating speed.
In a further method of improving the wetting property of a solid surface by attaching liquid steam such as water steam to the web, as disclosed in Japanese Patent Unexamined Publication No. 32923/1973, a relatively long time is required for causing a change in the wetting property. Accordingly, the attachment of steam is not sufficiently effected in such a zone as a decompression chamber of a slide coating apparatus in which the passage time is very short. Furthermore, in the case where condensed dew is attached to the web in the apparatus, fatal dot-like stains are undesirably produced.
As described above, in any conventional method, air entrainment could be prevented to a certain degree but its prevention was not satisfactory.